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MG Midget and Sprite Technical - Oil pressure

Bit of background:-
My 1100 A-series has done some 50k or so miles since rebuild. Runs as sweet as a nut, never had any issues. For as long as I can remember on start-up, capillary oil-pressure gauge would register 70psi. This would drop to 60 or so when hot @ about 3000rpm and at tickover somewhere between 30-40psi.
So a couple of weeks ago I changed the oil, filter and head gasket (the latter as I had a bit of a weep). Used same make/grade of oil, same make of filter that I've always used. On cold start-up I still get my 70psi, but now when hot I'm only getting 50 and 20-30 respectively. Engine is running fine, no unusual noises / rumbles / smoke / leaks etc.
My lack of belief in coincidences tells me that it's the oil change that was the cause. Perhaps I have a duff filter?
I'm not really bothered as oil pressure is clearly well within spec, more curious. Any thoughts anyone?
Simon
Simon Wood

" --- and head gasket (the latter as I had a bit of a weep) -- "

I'd say this was the more likely area. You may have a leak from the feed to the head. This could reduce pressure, and if it's leaking internally, you may not notice it for a while.

If you suspect the filter. Then remove it completely -- if you have the old style type. New oil, so you'll be fine for enough time to check the pressure when warm.
Lawrence Slater

Should have specified :- Fram paper element filter
Simon Wood

Lawrence, yes I wondered about the head gasket too. As I said, I have no obvious new oil leak, the level on the dipstick is steady, there's no evidence of oil in the cooling system and no clouds of smoke.
Correct me if I'm wrong but the oil feed from the block is a single hole at the back of the block. Where else could the oil possibly be going?
Simon Wood

Your new pressures look more normal to me then your old numbers... your old numbers way to high but with in reason, id think you will get better performance with the new numbers because your engine isnt working as hard

Granted and that said, you have an 1098, and im basing my knowledge on the 1275, so im just guessing and assuming

Prop
Prop and the Blackhole Midget

Did you use a differsnt oil brand then you normally do... im assuming 20/50 new or recycled oil

Basic Rule of thumb 10 psi per 1000rpm

Prop


Prop and the Blackhole Midget

Prop,
Been buying the same brand of oil for years from the same local motor factors:- Comma Classic 20/50. I agree with your observation about old v new pressures. I'm just curious.
Think that I will replace the filter and see what happens
Simon
Simon Wood

If your gauge is upstream of the oil filter then renewing the filter would remove any partial restriction to oil flow and decrease the pressure seen at the gauge.
Mark O

Good logic Mark, the implication being that if it is the new filter that is faulty then it is "not doing enough filtering" rather than causing an obstruction.

I suppose it could be that the felt seal under spring-loaded plate in the oil filter housing not sealing properly.

Thanks for the input
Simon
Simon Wood

This thread was discussed on 21/12/2015

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